From bd8b3c63d9a5113f6e241d61992ce6efd97b1f37 Mon Sep 17 00:00:00 2001
From: Jens Schauder <jschauder@vmware.com>
Date: Thu, 4 Nov 2021 07:18:40 +0100
Subject: [PATCH] Conditions are Expressions.

Selection of condition expressions yielding a boolean value is supported by some databases.

Closes #1007
Original pull request: #1079.
---
 .../data/relational/core/sql/Condition.java        |  2 +-
 .../core/sql/render/ComparisonVisitor.java         |  8 ++++----
 .../FilteredSingleConditionRenderSupport.java      | 12 +++++++-----
 .../core/sql/render/SelectRendererUnitTests.java   | 14 ++++++++++++++
 4 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/Condition.java b/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/Condition.java
index bacb5e84..956d2c68 100644
--- a/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/Condition.java
+++ b/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/Condition.java
@@ -23,7 +23,7 @@ package org.springframework.data.relational.core.sql;
  * @since 1.1
  * @see Conditions
  */
-public interface Condition extends Segment {
+public interface Condition extends Segment, Expression {
 
 	/**
 	 * Combine another {@link Condition} using {@code AND}.
diff --git a/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/ComparisonVisitor.java b/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/ComparisonVisitor.java
index 186c0601..476f1223 100644
--- a/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/ComparisonVisitor.java
+++ b/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/ComparisonVisitor.java
@@ -52,14 +52,14 @@ class ComparisonVisitor extends FilteredSubtreeVisitor {
 	@Override
 	Delegation enterNested(Visitable segment) {
 
-		if (segment instanceof Expression) {
-			ExpressionVisitor visitor = new ExpressionVisitor(context);
+		if (segment instanceof Condition) {
+			ConditionVisitor visitor = new ConditionVisitor(context);
 			current = visitor;
 			return Delegation.delegateTo(visitor);
 		}
 
-		if (segment instanceof Condition) {
-			ConditionVisitor visitor = new ConditionVisitor(context);
+		if (segment instanceof Expression) {
+			ExpressionVisitor visitor = new ExpressionVisitor(context);
 			current = visitor;
 			return Delegation.delegateTo(visitor);
 		}
diff --git a/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/FilteredSingleConditionRenderSupport.java b/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/FilteredSingleConditionRenderSupport.java
index 0a148c8d..8f23b672 100644
--- a/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/FilteredSingleConditionRenderSupport.java
+++ b/spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/FilteredSingleConditionRenderSupport.java
@@ -28,6 +28,7 @@ import org.springframework.util.Assert;
  * and delegate nested {@link Expression} and {@link Condition} rendering.
  *
  * @author Mark Paluch
+ * @author Jens Schauder
  * @since 1.1
  */
 abstract class FilteredSingleConditionRenderSupport extends FilteredSubtreeVisitor {
@@ -55,17 +56,18 @@ abstract class FilteredSingleConditionRenderSupport extends FilteredSubtreeVisit
 	@Override
 	Delegation enterNested(Visitable segment) {
 
+		if (segment instanceof Condition) {
+			ConditionVisitor visitor = new ConditionVisitor(context);
+			current = visitor;
+			return Delegation.delegateTo(visitor);
+		}
+
 		if (segment instanceof Expression) {
 			ExpressionVisitor visitor = new ExpressionVisitor(context);
 			current = visitor;
 			return Delegation.delegateTo(visitor);
 		}
 
-		if (segment instanceof Condition) {
-			ConditionVisitor visitor = new ConditionVisitor(context);
-			current = visitor;
-			return Delegation.delegateTo(visitor);
-		}
 
 		throw new IllegalStateException("Cannot provide visitor for " + segment);
 	}
diff --git a/spring-data-relational/src/test/java/org/springframework/data/relational/core/sql/render/SelectRendererUnitTests.java b/spring-data-relational/src/test/java/org/springframework/data/relational/core/sql/render/SelectRendererUnitTests.java
index bcb5a87a..fc4e6cd8 100644
--- a/spring-data-relational/src/test/java/org/springframework/data/relational/core/sql/render/SelectRendererUnitTests.java
+++ b/spring-data-relational/src/test/java/org/springframework/data/relational/core/sql/render/SelectRendererUnitTests.java
@@ -480,6 +480,20 @@ class SelectRendererUnitTests {
 		assertThat(rendered).isEqualTo("SELECT CAST(User.name AS VARCHAR2) FROM User");
 	}
 
+	@Test // GH-1007
+	void shouldRenderConditionAsExpression() {
+
+		Table table = SQL.table("User");
+		Select select = StatementBuilder.select( //
+				Conditions.isGreater(table.column("age"), SQL.literalOf(18)) //
+		) //
+				.from(table) //
+				.build();
+
+		final String rendered = SqlRenderer.toString(select);
+		assertThat(rendered).isEqualTo("SELECT User.age > 18 FROM User");
+	}
+
 	@Test // GH-968
 	void rendersFullyQualifiedNamesInOrderBy() {